As the reserves of conventional crude oils decline, there is an increasing demand for oil refineries to process moral contaminated heavy crude oil and residuum feedstocks. The cracking of these heavier fractions introduces numerous problems for refineries, as well as for catalyst manufacturers. For instance, these heavy hydrocarbon fractions frequently contain excessive quantities of organometallic contaminants which tend to be extremely detrimental to various catalytic processes that may be carried out during refining. Of the metallic contaminants, those containing nickel and vanadium are most common, although other metals such as iron are often present. These heavy fractions also have a high CCR(Conradson Carbon Residue).
Nickel and vanadium found in residual oils are in the form of complexes of porphyrin-like structures. The metals deposit almost quantitatively on the catalyst surface and act as dehydrogenating catalyst to produce excessive coke build-up and hydrogen formation in a catalytic cracker. These metal contaminants, particularly vanadium, have a deactivation effect over the catalyst. While there are potential advanced mechanisms for explaining such a deactivation by vanadium an irreversible destruction of the zoolite crystallinity of the catalyst. However, not only catalyst activity is affected, nickel tends to influence selectivity promoting dehydrogenation reactions leading to the formation of more hydrogen and coke. Excessive coke formation may upset the heat balance of the catalytic cracking unit and lead to metallurgical problems in the catalyst regenerator. Furthermore, the light gases that are produced may overload compressors associated with a fluid catalytic cracking unit.
For many years researchers have been looking for ways to mitigate the detrimental effects of nickel and vanadium. One technique has been to use a metal scavenger which reacts with the metal contaminants before the contaminants can react with the catalyst. In other words, a large dumping area is provided on the catalyst to accommodate the accumulating metals, and preferably separate the cracking function of the zeolite from the one of metal sink. In order to achieve this, it has been suggested to use a two range particle size with larger particles (180-100 .mu.m) which contain zeolite components with the cracking activity and smaller particles (30-60 .mu.m) with a matrix of kaolin and amorphous silica-alumina, and possibly other additives for minimizing SO.sub.x emissions and/or catalyzing oxidation.
Other suggestions include the use of basic alkaline earth oxides, such as MgO or CaO, to passivate vanadium by scavenging V.sub.2 O.sub.5 before it destroys the zeolite. Other proposed metal scavengers include magnesium silicates (sepiolite) and metakaolin microspheres.
Much research work has also been conducted and numerous patents have issued over the past twenty years on loading the catalyst with passivators. For instance, it has been known for many years that antimony compounds are effective passivators against the poisoning effect of nickel on cracking catalysts and that tin compounds are effective against the contaminating effects of vanadium.
For instance, Johnson et al U.S. Pat. No. 3,711,422 issued Jan. 16, 1973, describes the addition of an antimony compound to fresh catalysts or nickel and vanadium contaminated catalysts. The patent describes a variety of ways of incorporating the antimony compound, including incorporation in the feedstock, impregnating from aqueous solution, impregnating from non-aqueous solution and dry admixing.
Bertus et at U.S. Pat. No. 4,489,169 issued Dec. 18, 1984, describes a cracking catalyst which contains both antimony and tin compounds. The inventors describe a number of different ways of adding the antimony and tin compounds to the catalyst and state that the manner in which the conventional cracking catalyst is contacted with the antimony and tin modifying or treating agents is not critical.
It is the object of the present invention to provide a new and improved method of adding antimony and tin compounds to conventional cracking catalyst so as to achieve improved passivation.